Our aim is to identify, separate and characterize the voltage- and transmitter-activated ionic conductances underlying excitation in identified leech neurons in different species. During the last 10 years we have amassed a substantial amount of information on mechanisms of excitation in several leech neurons. One of the more important findings was that functionally identified neurons differ in their responsiveness to the classical Na- and K- channel blockers. The varying drug-sensitivities occurred in an orderly fashion suggesting that even closely related neurons posses specific sets of ionic conductances and are not identical. However, until the recent introduction in our laboratory of the two electrode voltage-clamp and patch clamp techniques, clear separation or kinetic analysis of ionic currents in leech neurons was completely lacking. There is no information on the properties of either voltage- or chemically activated channels in the leech or other annelids. The work should therefore provide interesting information regarding the phylogenetic evolution of ionic channels. Furthermore, because a large and growing body of information regarding neuronal function and biochemical content is afailable for various leech species, this preparation seems particularly suitable for the study of correlation between a cell's function and the properties of its ionic channels. We hope that findings on the biophysical properties of leech neurons will be not only of interest to the numerous workers who are presently studying leech behavior but will provide information of general significance on phylogenetic evolution of ionic conductances.